DISTRIBUTION IN SPACE. 145 



to different zoological provinces, if separated by any impass- 

 able barrier, such as a lofty chain of mountains. Owing to 

 their power of flight, the geographical distribution of birds 

 is much less limited than that of mammals; and many migra- 

 tory birds may be said to belong to two zoological provinces. 

 In spite of their powers of locomotion, however, birds are 

 limited by the necessities of their life to definite areas, and 

 a zoological province may be marked by its birds just as 

 well as by its quadrupeds. 



The geographical distribution of an animal at the present 

 day by no means necessarily coincides with its former ex- 

 tension in space. Many species are known which now 

 occupy a much more restricted area than they did formerly, 

 owing to changes in climate, the agency of man, or other 

 causes. Similarly, there are species whose present area is 

 much wider than it was originally. 



Zoological provinces must always have existed ; but those 

 of the present day by no means correspond with those of 

 former periods of the earth's history, but are, on the con- 

 trary, of comparatively recent origin. 



As regards the Mammals, the same forms are found occu- 

 pying the same regions in the later Tertiary period as they 

 do at present ; but the species are different. The distribu- 

 tion, therefore, of certain groups, dates back to a period an- 

 terior to the appearance of the now existing species of the 

 same groups. Thus, to take a single example, South Ame- 

 rica at the present day has amongst its many peculiar animals 

 none more characteristic than the Sloths and Armadillos 

 (Edentata). In late Tertiary time, however, Edentate ani- 

 mals were equally characteristic of the South American 

 fauna, though none of the living species then existed. Thus, 

 the modern Sloths are represented by the gigantic Megathe- 

 rium, Mylodon, and Megalonyx, and the little armour-plated 

 Armadillos find their ancient representative in the colossal 

 Glyptodon. It is to be remembered, however, that the law 

 thus indicated holds good for the later Tertiary period only, 



K 



